I searched some to find out what this is about and didn't find much. I know that canvas are used for fingerprinting the browser.
I thought that firefox is supposed to have some settings to block or allow canvas.
In the screenshot there is that picture icon beside the lock icon in the address bar and it seems that it could be the place to click to change the settings.
Problem is that the screenshot is shown less than a second and I wasn't able to click there.
As I read the screenshot the use of canvas is allowed for some reason.
I didn't find the reason and am wondering whether it is some trick amazon is using to make @firefox allow the access to canvas or if there is some flaw in firefox.
All in all it looks like I'm allowing canvas access (probably for fingerprinting?) contrary to my wish to deny it in favor of my #privacy.